“Biodiesel” typically refers to a diesel-equivalent, processed fuel derived from biological sources. Presently, one technique for producing biodiesel is based on transesterifying vegetable oils (e.g., soybean oil, rapeseed oil, and corn oil) or animal fats (e.g., tallow) in the presence of a catalyst to produce alkyl esters. The transesterification process can also produce glycerol, fatty acid, soap, and/or other undesirable contaminants. Even though the target specification for biodiesel varies from country to country, the total glycerol content in biodiesel is typically limited to about 0.25 to about 0.38 weight percent (wt %) or less.
Such a low glycerol requirement normally requires a two-step addition of the catalyst during the transesterification process. However, even with the two-step addition of the catalyst, it can still be difficult to meet the glycerol requirement when fats with high acid values, methanol with high moisture, and/or other low quality feedstock is used. For example, most commercial biodiesel production typically requires vegetable oil feedstocks with acid values of 3 or less, preferably 1 or less. Accordingly, there is a need for a more efficient and cost-effective technique for transesterifying triglyceride-containing fats to produce biodiesel or other alkyl esters.